An analytical procedure, based upon HPLC, has been used to determine
the degree of isomerization of retinol (vitamin A) in various dairy products.
In raw
milks, which are not subjected to thermal processing, there was no conversion
of the
predominant all-trans-isomers to cis-isomers in
samples from various species.
Pasteurized milks with mild heat treatment (high quality milk) had an average
13-cis[ratio ]all-trans ratio of 2·6%,
while pasteurized milk treated for 15 s at temperatures
ranging from 72 to 76°C had an average ratio of 6·4%. Milk subjected
to more severe
heat treatments had a higher degree of isomerization (UHT milk, 15·7%;
sterilized
milk, 33·5%), consistent with increased thermal conversion of the
retinol isomers. In
pasteurized and UHT cream, the increase in 13-cis-isomer was also
a consequence of
heat treatment (pasteurized cream, 3·0%; UHT cream, 14·4%).
The presence of cis
isomers in fermented milk suggests that fermentation processes, directly
or
indirectly, can induce cis–trans isomerization.
In the cheeses analysed, the extent of
retinol isomerization ranged from 7·6 to 35·0%. Our
results confirm the importance
of individual isomers in evaluating the vitamin A status of dairy products.